Magnetic compasses have been used by navigators for centuries, relying on the Earth's magnetic field to align a magnetized pivotable needle with the Earth's field to point to (magnetic) north. A variation on a needle is to have a centrally pivoted disc or card coupled to a magnet, a further variation is to immerse the disc in a transparent fluid to provide dampening. To minimize turbulence of the fluid around a disc and supports, the fluid is contained within a spherical transparent shell. As the vessel to be navigated is normally sea or air borne, then a compass therein must be able to accommodate rolling and pitching. This is achieved by a gimbal system that allows independent rotational movement in two horizontal axes.
The most common application of a magnetic compass is to ships and pleasure craft. These compasses are invariably of the type which are viewed from above, either directly or at an angle. It is to this compass type that the invention is directed.
There are several aspects of a conventional bowl compass that deserve attention, these include: the visual indicia to represent the orientation of a vessel relative to the compass; the viewing angle, the inertia of the compass card, the disposition of the magnetic elements to the pivot point; the means to accommodate expansion and contraction of an immersion fluid in response to temperature changes. In addition, conventional compasses are designed to be viewed from above or the side and a different design is necessary for a compass which is to be viewed from below. An ideal compass design would allow mounting in any orientation.
It has been found that hitherto each of these aspects exhibit shortcomings that compromise the performance of a compass. These are each discussed below.
The most important shortcoming is the interpretation by a viewer of the direction the craft is heading and the response necessary to maintain or align the craft to a new heading. Conventionally, a north-seeking magnet is secured under an opaque circular card on which is printed markings representing the 360 degrees of a circle and the cardinal points of a compass such as N, S, W and E. A reference or lubber line is fixed relative to the compass housing and is in close proximity to the outside of the card allowing readings to be taken of the direction of a vessel with respect to compass north.
The heading of a vessel to which a compass is installed is read with the aid of a reference line or lubber line close to the rim of the compass card. If the lubber line is positioned on the fore side of the compass such that when the N or 0 degrees mark is adjacent the lubber line, then the vessel is heading north. The shortcoming of this arrangement is that the corrective action to bring the vessel onto a course is not intuitive. For example if a northerly heading is desired and the lubber line indicates a heading of 020 degrees, the N direction on the card is anticlockwise of the lubber line and many helmsman especially novice ones are inclined to steer the vessel clockwise as if to bring the N mark over to the lubber line. This tendency arises because to an observer it seems that the compass card has moved or rotated, when in fact it has remained stationary and it is the orientation of the vessel that has changed.
If the lubber line is positioned on the aft side of the compass, the graduations on the card will 180 degrees displaced, N where S is and E where W is. The compass remains counter intuitive even though the lubber line is viewed ahead of the card. As in the example above, if the heading is 020 degrees, the north direction, which is 20 degrees to the left (port) will be shown 20 degrees to the right (starboard) of the lubber line.
The displacement of the lubber line from the card allows for parallax error to be present when not reading the compass from directly behind. Two observers side by side will read different headings to each other.
A different presentation of the indicia could offer a more intuitive recognition of corrections necessary to align a vessel to a specified heading. This aspect is the principal element of the invention and will be discussed further in a following section.
Additional aspects of compass design are relevant and are discussed herewith. The centre of the card has a pivot point about which the card rotates and can tilt a few degrees relative to a pivot support which is gimbal mounted allowing tilting freedom. The magnets are invariably secured below the pivot point so that the centre of mass of the card-magnet system is below the pivot point thereby keeping the card in a substantially horizontal plane.
A compass card and associated supports are housed in a spherical shell which is filled with transparent oil such as paraffin. This primarily dampens the movement of the card in response to vibrations and sharp movements. Since the compass will experience wide ranges of temperature, an elastic diaphragm is incorporated in a wall of the spherical chamber, said diaphragm accommodating any changes in fluid volume.
The compass card has hitherto been made as thin and as light-weight as possible. This feature has been to satisfy a recognized performance test which measures the time it takes for a card to redirect itself when misaligned for example by an external magnet. Clearly minimizing the angular inertia of the compass card better achieves the measure of the test, however it does not reflect the disturbances experienced in a real application. In practice, if the compass support is disturbed, this disturbance will be transferred through to the card via both pivot friction and fluid friction, for which a light-weight card will respond detrimentally.
The objective of this invention is to overcome the above shortcomings either individually or preferably—collectively.